1. E. Soldatov17.10.2011 Tight photon efficiency study using FSR photons from Z  ll  decays E.Yu.Soldatov* *National Research Nuclear University “MEPhI” Outline: 1.Selection criteria 2.Puries of samples 3.Tight cut efficiency from Z->ee  /  4.Background estimation method Photon ID efficiencies meeting

2. E. Soldatov Introduction A “minor” problem: a small production cross section. What do we want? The main idea is to obtain a photon sample with maximum purity with the method decoupled from the standard analysis methods. Why do we want this? Studies of the ATLAS detector performance with tagged photons and first of all study and optimization of the tight cut criteria. Very important for analysis all photon containing processes. The best source of such kind of photons is a production of FSR photons from Z-lepton decay: № 2№ 2 Photon ID efficiencies meeting17.10.2011

3. E. Soldatov № 3№ 3 Introduction: Samples&Preselections 5M sample of Z-boson decays to electrons and muons from mc10 was used (ATHENA Release 16.6.4.3) : Electron channel: MC: mc10_7TeV.106046.PythiaZee_no_filter.merge.AOD.e574_s933_s946_r2399_r2300/ Data: 2011 Periods D-L5 (Egamma stream) – 2.14 fb -1 Muon channel: MC: mc10_7TeV.106047.PythiaZmumu_no_filter.recon.AOD.e574_s933_s946_r2399/ Data: 2011 Periods D-K (Muons stream) – 2.03 fb -1 For all particles the initial requirements are: E T >5 GeV for photons, P T >20 GeV for electrons and muons, |  |<2.47, excluding the crack region (1.37<|  |<1.52) between the barrel and end-cap calorimeters. deltaR(l  )>0.2 && for leptons applied preselection consistent with EPS recommendations Photon ID efficiencies meeting17.10.2011 MCTruthClassifier used for truth matching

4. E. Soldatov Introduction: Selection № 4№ 4 1. Mass of two leptons should have a missing part e.g. < Z bozon mass For electron channel 60 < m( ee ) < 83 GeV, and for muon channel 40 < m(  ) < 82 GeV A kinematic approach of the photon selection 2. And 3 body mass should correspond to a mass of Z bozon For electron channel 80 < m( ee  )< 94 GeV and for muon channel 81 < m(  ) < 95 GeV Kinematics approach is based on a simple criteria of the event selection. Background -ISR -Brem -Jets - FSR Signal Three body mass spectrum (Z   ) 17.10.2011Photon ID efficiencies meeting 3. Isolation: Etcone20_ptcorrected<5 GeV && Nucone20=0.

5. E. Soldatov Tight cut efficiency study MCTruthClassifier check of the background № 5№ 5 Photon ID efficiencies meeting17.10.2011 There are many of FSR in bkg!!!

6. E. Soldatov Tight cut efficiency study MCTruthClassifier check of the background after tight cut № 6№ 6 Photon ID efficiencies meeting17.10.2011 There are many of FSR in bkg!!!

7. Photon purity after all cuts applied E. Soldatov Differential purity for each bin. № 7№ 7 Photon ID efficiencies meeting17.10.2011 Purity=Signal/(Signal+Background) Et [GeV] Z->ee  Et [GeV] Unconverted photons Converted photons Z->ee  Z->  Syst=1.0% Syst=0.5% Syst=0.3% Syst=0.1%

8. E. Soldatov № 8№ 8 Signal/background ration is much better at for higher photon energies. Introduction 3 body mass spectrum with constrains on the mass of two leptons (Data and MC) 17.10.2011 Z->ee  E T (  )>15 GeV E T (  )>5 GeV Photon ID efficiencies meeting (Electrons pass Medium cut)

9. E. Soldatov Introduction Photon spectrum after selection of the signal area applying 3 body mass cut 80 < m(ee  )< 94 GeV 81 < m(  ) < 95 GeV № 9№ 9 17.10.2011 Z->ee  Z->  Photon ID efficiencies meeting

10. E. Soldatov Tight cut efficiency study Efficiency of robust tight cut on photons vs Et spectrum № 10 Photon ID efficiencies meeting17.10.2011 E T [GeV] Z->ee  - Sgn sample MC - Data 2011  - Sgn sample MC - Data 2011  E T [GeV] Converted photons Unconverted photons

11. E. Soldatov Tight cut efficiency study Efficiency of robust tight cut on photons vs Et spectrum № 11 Photon ID efficiencies meeting17.10.2011 E T [GeV] - Sgn sample MC - Data 2011  E T [GeV] Converted photons Unconverted photons - Sgn sample MC - Data 2011  - Sgn sample ffMC - Data 2011  - Sgn sample ffMC - Data 2011  Z->  Fudge factor correction Nominal MC

12. E. Soldatov Data driven background estimation Work of the background estimation method on real data № 12 Photon ID efficiencies meeting17.10.2011 E T (  )>5 GeV, E T (  )<10 GeV Syst~2-3% Z->ee  N (tracks in cone20)

13. E. Soldatov Conclusions Correction of truth matching for signal selection has been done. The purity became larger than 98% for 15 GeV photon sample. Monte Carlo simulation has been compared with the 2011 Data (periods D-K with ~2 fb -1 of integral luminocity). To improve the agreement between simulation and data background estimation method from data has been proposed and implemented. Tight cut efficiency has been estimated using two methods. MC and Data descrepancy became very small after fudge factor MC correction. № 13 Photon ID efficiencies meeting17.10.2011

14. E. Soldatov Back-up slides № 14 Photon ID efficiencies meeting17.10.2011

15. E. Soldatov Tight cut efficiency study Multiplicity for data № 15 Photon ID efficiencies meeting17.10.2011

16. Data driven background estimation E. Soldatov Crosscheck on a larger statistics! In order to obtain more background photons under in a signal sample less stringent kinematic conditions where taken: 40 < m(  ) < 88 GeV 75 < m(  ) < 105 GeV Excellent agreement! E T (  )>5 GeV N (tracks in cone20) Number of tracks in cone 0.2 around photon vector in ID. Again the black line shows the background estimate using extrapolation method described above. № 16 Photon ID efficiencies meeting17.10.2011

17. E. Soldatov MC: Study of the pure background sample. Spectrum of background photons from the signal sample after 3 body mass CUT applied E T [GeV] Spectrum of background photons from the background sample Applying 3 body mass cut we deform a bit the photon spectrum. Does it affect the background estimation? Background photon spectrums Cut Et (in cone 0.2)<3 GeV № 17 17.10.2011Photon ID efficiencies meeting